Changing the rear end ratio will effect the speedometer reading and require recalibration. The speedometer is calibrated for a specific rear axle ratio such that a specific number of turns on the driveshaft will result in a specific speed based on the speed of the rear wheel. By changing the differential ratio the tire speed (rpm) will change in relations to driveshaft speed (rpm). Said in more understandable way, the distance covered by the vehicle in in the same amount of time will be less or more based on the ratio change as Bob mentioned above. Since speed is a measure of distance per unit of time, whenever you change the time required to cover a distance or the distance itself the actual speed is altered. In addition, it should be noted that calibrating the needle to tweak it to the proper speed is able to be done when the speedometer gears are correct for the ratio of the differential. You will notice that the speedometer has numbers on the face, which indicate the differential ratio it will work correctly for. Without having that correct the odometer reading will also be inaccurate. The miles registered under the different ratio will be less or more based on which way the differential ratio was changed. As indicated by others in the above posts, there are some good websites on this. Nisonger in Mamaroneck can answer what needs to be changed to correct for the change in differential ratio. I hope I didn't confuse the issue.